Vacuum plasma processors deposit materials on and etch materials from workpieces that are typically semiconductor, dielectric and metal substrates. A gas is introduced into a vacuum plasma processing chamber where the workpiece is located. The gas is ignited into a plasma in response to an r.f. electric or electromagnetic field. The r.f. field is provided by a reactive impedance element, usually either an electrode array or a coil which couples both magnetic and electrostatic r.f. fields to the gas. The reactive impedance element is connected to an r.f. source having a frequency and sufficient power such that the gas is ignited into an r.f. plasma. Connections between the source and the reactive impedance element are usually by way of a relatively long coaxial cable, connected directly to the r.f. source and a matching network. The matching network includes a pair of variable reactances, adjusted to (1) control tuning of the reactive impedance element and its load, including the r.f. plasma, relative to the source frequency and (2) match the impedance of the source to the load it is driving. When a match is reached the impedance seen looking into the r.f. source output terminals is about the same as the impedance seen by the r.f. source looking into the cable.
The load seen by the source is subject to substantial variations. The load is a relatively high impedance prior to ignition of the gas into the r.f. plasma state. In response to the plasma being ignited, the load impedance drops substantially due to the presence of the charge carriers, i.e., electrons and ions, in the excited plasma. The ignited plasma impedance also changes substantially due to variations in the plasma flux, i.e. the product of the plasma density and the plasma charge particle velocity. Hence, matching the source to the load to provide efficient transfer of power from the source to the load is somewhat difficult.
As the two matching network reactances are simultaneously varied along a trajectory to achieve matching, the plasma sometimes becomes unstable and the flux thereof varies in amplitude unpredictably. We have discovered that when the plasma is unstable the r.f. current changes in amplitude in a manner similar to the way the r.f. current of a radio transmitter changes during audio modulation. The r.f. is modulated in first and second frequency bands, respectively of 2 kHz-20 kHz and 50 kHz-200 kHz. Accordingly an unstable plasma is occasionally referred to herein as a modulated plasma. The instability varies the plasma flux so there is undesirable plasma activity on the workpiece. The plasma can become so unstable that it flickers on and off.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for controlling a plasma of a vacuum plasma processor to enable uniform, predictable workpiece characteristics to be attained.
Another object of the invention is to provide a method of and apparatus for controlling reactances of a matching network connected between an r.f. source and a vacuum plasma processor in such a way that instability of the plasma, also known as plasma modulation, is, to a large extent, avoided while achieving matching.
Another object of the invention is to provide a new and improved method of and apparatus for detecting plasma instability in a vacuum plasma processor.